Dehydrator Onion Bulb Weight and Water-soluble Carbohydrates ...

J. AMER. SOC. HORT. SCI. 124(6):581–586. 1999.
Dehydrator Onion Bulb Weight and Water-soluble
Carbohydrates before and after Maturity
Arthur D. Wall, 1 Marisa M. Wall, 2 and Joe N. Corgan 3
Department of Agronomy and Horticulture, Box 30003, MSC 3Q, New Mexico State University, Las Cruces,
NM, 88003
ADDITIONAL INDEX WORDS. Allium cepa L., nonstructural carbohydrates, soluble solids, fructans, sugars, bulb mass, bulb size,
high performance liquid chromatography
ABSTRACT. Onions (Allium cepa L.) with ≥18% bulb dry weight are dehydrated and used for spices and food ingredients. Bulb
weight characteristics and water-soluble carbohydrates (WSC) of two commercial dehydrator cultivars, GS02 and GS04,
and a breeding population, NM9335, were studied before and after maturity to observe phenotypic traits that may be useful
for selection during breeding programs, and to study dehydrator onion carbohydrate physiology. At maturity, NM9335,
GS02, and GS04 bulbs had 11.9 ± 0.33%, 18.6 ± 0.27%, and 19.4 ± 0.40% dry weight, respectively. Mature GS04 plants had
76.5 ± 0.01% of whole plant dry weight in bulbs, which is an extraordinarily high crop harvest index. NM9335 bulbs had
higher fresh (hydrated) weight than bulbs of GS04 and GS02, but bulbs in all populations accumulated similar amounts of
dry weight. Bulb percent dry weight before maturity did not indicate percent dry weight at maturity in the high-solids
commercial onion cultivars. Bulb percent dry weight declined slightly after maturity in all populations. Glucose, fructose,
and sucrose were relatively low, and fructans with degree of polymerization ≥6 were relatively high in GS04, but the converse
was observed in NM9335. Relative amounts of GSO4 bulb fructan increased sequentially, in order of rank, from DP4 to DP6,
but the converse was observed for NM9335.
White-skinned onion cultivars are dehydrated and used as primary
ingredients in food seasonings and spices (American Spice
Trade Association, 1993; Fenwick and Hanley, 1990). Dehydrator
onion cultivars with ≥18% bulb dry weight (solids) content are
distinct from fresh market types, with ≈5% to 9% bulb solids, and
storage types, with ≈8% to 12% bulb solids (Darbyshire and Steer,
1990). Onion solids increase from inner to outer scales, and from the
base to the apex of onion bulbs and plants (Darbyshire and Henry,
1978). Onion water soluble carbohydrate (WSC), or nonstructural
carbohydrate, forms 80% to 87% of bulb dry weight, and is
comprised of glucose, fructose, sucrose, and fructans. Onion fructans
are fructose polysaccharides (oligosaccharides), with degree of
polymerization (DP) from 3 to 15 subunits, depending on population
(Darbyshire and Henry, 1979; Darbyshire and Steer, 1990;
Suzuki and Cutcliffe, 1989). Other Allium species can accumulate
fructans with DP > 15 (Darbyshire and Henry, 1981). The most basic
fructans are DP3 trisaccharides, synthesized by transfer of a fructose
molecule to sucrose, to form two isomers: 1 F -fructosylsucrose and
6 G -fructosylsucrose. Longer DP fructans form by addition of fructose
subunits to ≥DP3 fructan (Endleman and Jefford, 1968; Henry
and Darbyshire, 1979). However, Cairns (1993) states that previous
reports on fructan synthesis require reappraisal, and that fructan
synthesis enzyme chemistry is still poorly understood.
Onion dry weight content is usually measured indirectly by using
a refractometer to assay for soluble solids content (SSC) in onion
juice (Dowker, 1990; Pike, 1986). Sinclair et al. (1995) report a 0.99
correlation between refractometer SSC readings and actual dry
Received for publication 17 Sept. 1998. Accepted for publication 17 Aug. 1999.
We thank Cindy Waddell for support in the HPLC analysis, Jose Luis Mendoza,
Helen Redden, Melodie Borden, and M.R. Doyle for research assistance, and
Gilroy Foods, Inc. for providing commercial dehydrator onion seed used in this
study. The research was supported by the New Mexico Agricultural Experiment
Station, The New Mexico Dry Onion Commission, and the Jose Fernandez Chair
for Crop Production.The cost of publishing this paper was defrayed in part by the
payment of page charges. Under postal regulations, this paper therefore must be
hereby marked advertisement solely to indicate this fact.
1 Graduate research assistant.
2 Associate professor.
3 Professor emeritus and Jose Fernandez chair for crop production.
J. AMER. SOC. HORT. SCI. 124(6):581–586. 1999.
weight content in a study of 49 diverse onion cultivars. Several other
studies also have proven that SSC is an effective selection criterion
for dry weight content (Mann and Hoyle, 1945; Nieuwhof et al.,
1973). Bulb SSC directly indicates dry weight content, but does not
indicate profiles of constituent WSC. Nondestructive gravimetric methods
and near-infrared spectrophotometry also can be used to test for SSC
or dry weight in onions (Birth and Dull, 1985; Kehr, 1952).
The present work was conducted to characterize the accumulation
of dehydrator onion bulb fresh and dry weight before maturity,
and the retention of bulb fresh and dry weight after maturity, by
onion populations differing in dry matter percentage (SSC). Postmaturity
treatments were included to study these traits in onions
during periods when harvest was delayed past maturity, and during
summer bulb dormancy. Ratios and proportions (profiles) of constituent
WSC were determined in a commercial cultivar and a
breeding population before and after maturity using high-performance
liquid chromatography (HPLC), to gain a better understanding
of WSC metabolism in dehydrator onion bulbs. The relative
differences in WSC profiles over various maturities may serve as
phenotypic markers for onion breeding and physiology research.
The contribution of leaves and pseudostem to dry matter partitioning
was studied in a high solids onion cultivar to observe weight
accumulation and translocation during the bulb-fill period. Previous
research has described onion bulb weight characteristics and WSC
at maturity in cultivars that differ in dry matter percentage (Darbyshire
and Henry, 1978, 1979; Darbyshire and Steer, 1990). However,
presently there are no reports on dehydrator onion bulb carbohydrate
and dry matter accumulation before maturity, or retention in
bulbs that remain rooted in situ after maturity. Additionally, the
HPLC method used in this study is a new modified procedure that
provides relatively efficient qualitative analysis of onion WSC,
which could be especially useful for onion breeding and processing
quality control programs.
Materials and Methods
POPULATIONS. Two intermediate-day dehydrator onion cultivars
‘GS02’ and ‘GS04’, and an intermediate-day open-pollinated N.M.
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State Univ. breeding line, ‘NM9335’, were chosen for this study.
The GS02 and GS04 cultivars are proprietary cultivars that are used
in commercial dehydrator onion production in California. The
NM9335 dehydrator onion breeding population was an openpollinated
F7 generation that originated from the cross of ‘Ben
Shemen’ x ‘Southport White Globe’ and had one previous selection
cycle for SSC. The NM9335 population was well adapted to the
experimental location, which was indicated by vigorous growth,
large bulbs, and disease resistance. The GS02 and GS04 cultivars
had relatively small bulbs that had more symptoms of pink root
(Phoma terrestris Hansen) and fusarium basal rot [Fusarium
oxysporum f. sp. cepae (Hans) Snyder and Hansen], relative to
NM9335.
FIELD EXPERIMENTS. Experiments were conducted in 1994 and
1995, and were located at the Fabian Garcia Agricultural Science
Center in Las Cruces, N.M. A split-plot experimental design with
six replications was used for bulb characteristic studies before and
after maturity. Onion populations were planted in rows to form main
plots, and maturity treatments were applied as subplots within rows.
A randomized complete block design with six replications was used
for plant-weight partitioning studies of the GS04 cultivar. Plots were
established 1.1 m long, with a 1-m row spacing, and with four
equally spaced lines of plants direct-seeded in a 76-cm-wide bed on
26 Jan., both years. Seedlings were hand thinned to 6 cm between
plants at four weeks after emergence. The crop was managed
according to standard practices for southern New Mexico, with
water and nitrogen applied using drip irrigation (Bailey and Corgan,
1986; Corgan and Kedar, 1990). Plots were considered mature when
≈80% of the tops had fallen. Predicted and actual maturity closely
coincided both years. The NM9335 population matured on 15 July
1994 and 10 July 1995. The GS02 cultivar matured on 13 July 1994
and on 7 July 1995, and the GS04 cultivar matured on 7 July 1994
and on 20 July 1995. Onion bulbs were hand harvested from within
the middle 0.9 m of the plot. Onion roots and tops were clipped at
harvest, diseased bulbs were discarded, and harvested bulbs were
stored temporarily in paper bags at ambient air temperature (≈25
°C). Bulbs were left rooted in place with tops attached, for postmaturity
harvest treatments. During 1994, treatments were designed to
harvest plots at 45, 30, and 15 d before maturity, at maturity, and at
15, 30, and 45 d after maturity. The 45 d treatments were omitted
from the 1995 experiments.
On the day of harvest, and just before testing, bulbs were washed
and dried to remove soil and dry scales, top and root stubble were
removed, and fresh yield data was obtained from these samples.
Samples of GS04 plants were prepared for weight partitioning
studies by cutting 10 plants from each replication into leaves,
pseudostems, and bulbs. Percent dry weight was determined by
hand chopping the 10-bulb subsample and the plant parts into coarse
pieces (≈2 to 3 cm), which were then dried at 100 °C for 1 h, and then
at 70 to 75 °C for 48 h, in a forced air convection oven. Dried samples
were frozen at –10 °C subsequent to WSC analysis using HPLC.
Average bulb fresh weight was calculated by dividing plot fresh
yield by the number of bulbs per plots. Average bulb dry weight was
calculated by multiplying average bulb fresh weight by average bulb
percent dry weight.
WATER-SOLUBLE CARBOHYDRATE ANALYSIS. HPLC analysis was
conducted according to the modified methods of Suzuki and Cutcliffe
(1989) and Scobell et al. (1977), and four replications of each field
treatment were analyzed. Dried bulb samples were further prepared
by chopping and mixing in a food processor, and then 20 to 30 g of
these flakes were ground into powder to pass through an 80 mesh
screen. Powder was mixed with triple distilled water (1 g/100 mL)
and shaken for 1 h at 25 °C. Solutions were vacuum filtered through
#40 filter paper, and then refiltered into HPLC vials through a 45 μm
membrane filter disc. The vials were sealed and then immediately
stored at –10 °C before HPLC analysis.
Carbohydrates were analyzed by injecting 20 μL of sample
solution into a Hewlett-Packard 1090 series liquid chromatograph
with deionized, distilled, degassed water as the mobile phase, and an
ion moderated partition column (Bio-Rad Aminex HPX-42A, Hercules,
Calif.) as the stationary phase, followed by a Hewlett Packard
1047A refractive index detector. A flow rate of 0.6 mL·min –1 and a
column temperature of 85 °C were used.
Glucose, fructose, and sucrose standard sugars were used to
calculate column retention times for these onion WSC. Highfructose
corn syrup oligosaccharide standards indicated the column
was detecting polysaccharides in sequential rank of degree of
polymerization (DP), which infers that onion fructans also elute in
sequential rank of DP. Quantitative analysis of onion WSC was not
possible because fructan standards were not available. Previous
research on onion WSC confirms that glucose, fructose, sucrose and
fructans are the only constituent WSC carbohydrates found in
Allium cepa L., and that fructans elute in sequence, based on rank of
fructan DP (Darbyshire and Henry, 1978, 1979). Based on these
assumptions, relative amounts of WSC were estimated from peak
Fig. 1. Bulb weight characteristics of three dehydrator onion populations before and
after maturity, averaged over 1994 and 1995.
582 J. AMER. SOC. HORT. SCI. 124(6):581–586. 1999.
weight

areas of chromatograms, and WSC identification was based on
column retention time.
Results and Discussion
Analysis of variance and homogeneity of variance tests indicated
that data should be combined and averaged for 1994 and 1995.
BULB WEIGHT CHARACTERISTICS BEFORE AND AFTER MATURITY.
NM9335 bulbs accumulated fresh weight at a faster rate before
maturity than bulbs of GS02 and GS04. NM9335 bulb fresh weight
was much higher than GS04 and GS02 bulbs at maturity. However,
because NM9335 had lower bulb percent dry weight than GS02 or
GS04, all populations accumulated similar amounts of bulb dry
weight (Fig. 1). Bulb fresh weight remained about the same after
maturity for each population, although bulbs of GS04 lost fresh
weight 30 d after maturity (Fig. 1). NM9335 bulb fresh weight
increased ≈8% at 15 d after maturity, which may indicate that the
harvest rating of ≈80% tops fallen was premature for this population
(Fig. 1).
Percent dry weight of NM9335 bulbs was constant, and GS02
and GS04 percent dry weight increased before maturity (Fig. 1),
which indicates that bulb percent dry weight before maturity did not
indicate percent dry weight at maturity in the high-solids dehydrator
onion cultivars. Lancaster and Kelly (1984) studied percent dry
weight in bulbs of ‘Southport White Globe’ and ‘Spartan Sleeper’,
with ≈13% bulb dry weight, and reported that percent dry weight
before maturity accurately predicted percent dry weight at maturity,
by testing the third inner scale. Their study was not replicated, and
Fig. 2. High-performance liquid chromatogram of soluble carbohydrates for a 10bulb
sample of dehydrator onion breeding population NM9335 at maturity in
1994. DP denotes the degree of polymerization of fructan polysaccharide.
J. AMER. SOC. HORT. SCI. 124(6):581–586. 1999.
data were reported based on single bulb samples, so their results
were more likely a reflection of variability for bulb percent dry
weight within the study populations rather than an effect of bulb
maturity on percent dry weight.
At maturity, bulbs of NM9335, GS02 and GS04 had 11.9% ±
0.33%, 18.6% ± 0.27% and 19.4% ± 0.40% dry weight, respectively.
Bulb percent dry weight decreased 0.5% to 1% at 30 d after
maturity for each population (Fig. 1). Bulb percent dry weight losses
could perhaps be due to carbohydrate respiration, balanced by
increases in percent dry weight due to bulb desiccation.
ONION WATER SOLUBLE CARBOHYDRATES BEFORE AND AFTER MA-
TURITY. Profiles of WSC were distinctly different between the
intermediate-solids NM9335 breeding population (Fig. 2) and the
high-solids GS04 commercial cultivar (Fig. 3). Differences between
the higher DP fructans were not resolvable because of
coelution at the same column retention time, which was indicated by
chromatograms with an indistinct secondary peak preceding the
main peak of the highest DP fructan (Figs. 2 and 3).
At each maturity, GSO4 had a much larger proportion of bulb dry
weight in WSC, relative to NM9335, and most of the WSC was
≥DP6 fructan (Figs. 4 and 5). Fructose, glucose and sucrose were
relatively low in GS04, and were relatively high in NM9335, which
concurs with results of Darbyshire and Henry (1979), who also
studied WSC of intermediate- and high-solids onion populations.
Fructose and sucrose levels were variable across all maturity dates
in GS04 bulbs, whereas, glucose levels decreased approaching
maturity, and then increased slightly after maturity (Fig. 4). Propor-
Fig. 3. High-performance liquid chromatogram of soluble carbohydrates for a 10bulb
sample of commercial dehydrator onion cultivar GS04 at maturity in 1994.
DP denotes the degree of polymerization of fructan polysaccharide.
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Fig. 4. Proportions of soluble carbohydrates from bulbs of commercial dehydrator
onion cultivar GS04 before and after maturity, averaged over 1994 and 1995. Data
for the 45 d postmaturity treatment is from 1994 only.
Fig. 6. Proportions of soluble carbohydrates from leaves of commercial dehydrator
onion cultivar GS04 before maturity, averaged over 1994 and 1995. Data for the
–45 d prematurity treatment is from 1994 only.
Fig. 5. Proportions of soluble carbohydrates from bulbs of dehydrator onion
breeding line NM9335 before and after maturity, averaged over 1994 and 1995.
Data for the 45 d postmaturity treatment is from 1994 only.
Fig. 7. Proportions of soluble carbohydrates from pseudostems of commercial
dehydrator onion cultivar GS04 before maturity, averaged over 1994 and 1995.
Data for the –45 d prematurity treatment is from 1994 only.
584 J. AMER. SOC. HORT. SCI. 124(6):581–586. 1999.

tions of DP3, DP4, and DP5 fructan were generally consistent
before and after maturity, whereas most WSC accumulated as ≥DP6
fructan before maturity in GS04 bulbs. Fructan ≥DP6 was measured
at various levels after maturity in GS04 bulbs (Fig. 4). Mean
standard errors of ≥DP6 fructan in GS04 bulbs were high. This may
have been due to environmental variability, because laboratory
replications were derived from replicated field plots. Also, sample
variability would likely have been improved if the entire 10 bulb
subsample from each replication had been ground to pass an 80
mesh screen.
Proportions of fructose, glucose and sucrose were variable in
NM9335 bulbs across maturity treatments (Fig. 5). Fructan levels
increased in order from DP3 to DP6 in GS04 bulbs (Fig. 4), but
decreased in order from DP6 to DP4 in NM9335 (Fig. 5), before and
after maturity. This indicates that GSO4 accumulated more high-DP
fructan, and NM9335 more short-DP fructan. These differences in
WSC profiles provide interesting contrasts for further research on
fructan synthesis and carbohydrate biochemistry, especially for the
enzyme fructose–fructosyltransferase (polymerase) that is reported
to control the DP of fructan molecules (Cairns, 1993; Henry and
Darbyshire, 1978, 1979). Darbyshire and Henry (1979) also reported
that high-solids onion WSC was mostly in the form of higher
DP fructan, and with relatively low amounts of glucose, fructose and
sucrose.
Differences between populations for WSC profile lead to relevant
questions for onion breeders. Breeding populations under
development for dehydrator characteristics are often screened for
bulb percent dry weight directly by dehydration, or indirectly by
using a refractometer to test for soluble solids in onion juice
(Dowker, 1990; Pike, 1986). Refractometer data can have up to a
0.99 correlation with dry weight (Sinclair, 1995), but do not reflect
proportions of constituent WSC. Dehydrator onion cultivars with
(%)
Fig. 8. Proportions of plant fresh weight in bulbs, pseudostems, and leaves of
commercial dehydrator onion cultivar GS04 before maturity, averaged over 1994
and 1995. Data is expressed as percent of total plant weight. Data for the –45 d
prematurity treatment is from 1994 only.
J. AMER. SOC. HORT. SCI. 124(6):581–586. 1999.
high bulb solids may possess better processing characteristics, such
as whiter color after drying, better organoleptic properties, or better
milling qualities, relative to onion populations with lower bulb
solids, which also are relatively lower in fructan and higher in
sucrose, fructose and glucose. Simple sugars (mono- and disaccharides)
in onions may lead to greater browning, or caramelization
reactions, during the dehydration process. In these studies GS02 and
GS04 bulbs with ≈18.5% to 19.5% solids dried with a whiter color
than bulbs of the ≈12% solids, NM9335, under similar drying
conditions, based on subjective observation. NM9335 bulbs were
relatively low in fructan and high in the sugars, fructose, glucose and
sucrose, compared to GS04 bulbs. Presently, there are no reports on
how recurrent selection for bulb dry weight or SSC will affect WSC
profiles, or if recurrent selection will increase fructan content,
without a proportional increase in fructose, glucose and sucrose.
WATER SOLUBLE CARBOHYDRATE CHARACTERISTICS OF GS04 BULBS,
LEAVES, AND PSEUDOSTEMS BEFORE MATURITY. Water soluble carbohydrate
increased from the base to the top of GS04 plants before
maturity, which agrees with the report of Darbyshire and Henry
(1978). Very small amounts of DP3 to DP5 fructan were detected in
GS04 pseudostems, and trace amounts were found in leaves before
maturity (Figs. 6 and 7). Leaf and pseudostem WSC were mostly
simple sugars, which decreased as the proportion of dry weight
before maturity (Figs. 6 and 7). Pseudostem WSC increased from 45
to 30 d before maturity, and then declined from 30 d before maturity
to maturity (Fig. 7). The study period coincided with the last six
weeks of bulb development, as onion plants translocate photosynthetic
carbon to bulbs before bulb summer dormancy.
Darbyshire and Steer (1990) also reported that levels of sugar in
leaves, pseudostems, and bulbs increase with age until the final
stages of bulb development. Darbyshire et al. (1979) reported that
fructose levels in leaves and pseudostems were generally higher
than glucose or sucrose, but diurnal fluctuation affected the amount
(%)
Fig. 9. Proportions of plant dry weight in bulbs, pseudostems, and leaves of
commercial dehydrator onion cultivar GS04 before maturity, averaged over 1994
and 1995. Data is expressed as percent of total plant weight. Data for the –45 d
prematurity treatment is from 1994 only.
585

and types of sugars in onion leaves and pseudostems. Several studies
have shown that fructan accumulation in leaf bases of young plants
coincides with initiation of bulbing in onions (Darbyshire and
Henry, 1978; Henry and Darbyshire, 1978). Changes in WSC
composition in leaves and pseudostems may coincide with the
cessation of new leaf emergence, which denotes the end of vegetative
growth and acceleration of active bulb enlargement in onions
(Brewster, 1990).
WEIGHT PARTITIONING IN GS04 PLANTS BEFORE MATURITY. GS04
bulb weight increased, and pseudostem and leaf weight decreased
before maturity (Figs. 8 and 9). The proportion of whole plant dry
weight in bulbs increased from 31.8 ± 0.01 to 76.5 ± 0.01% in the
45 d period before maturity, demonstrating that the GS04 dehydrator
onion cultivar was very efficient at converting plant dry weight
into net yield, which is commonly expressed as the crop harvest
index.
Conclusions
The commercial dehydrator cultivars and the breeding population
had distinctly different bulb weight and WSC characteristics.
Bulb traits and WSC profiles of the commercial cultivars provide
standards for progress in the breeding population. The HPLC
method is useful to evaluate potential parents for a breeding program,
to typify breeding populations, and to evaluate progress in
selecting for WSC content. The method ideally would be improved
to resolve differences between higher DP fructans, to reduce sample
variability, and to develop fructan standards to quantify onion WSC.
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